In the recent FTTH system, a passive optical access network called GE-PON (Gigabit Ethernet-Passive Optical Network) is used.
FIG. 5 shows a diagram of the structure of a GE-PON.
As shown, the GE-PON comprises an OLT (Optical Line Terminal) 51, a splitter 52, and ONUs (Optical Network Units) 53. The GE-PON forms a tree structure, with the OLT 51 being at the top.
A downlink signal from the OLT 51 is reproduced at the splitter 52, and is transmitted to all the ONUs 53. Each ONU 53 refers to the LLID (Logical Link ID) in the preamble, and discards the frames other than the one directed thereto. Meanwhile, as uplink signals from the ONUs 53 are transmitted in respective periods of time allocated by the OLT 51, the uplink signals are integrated at the splitter 52 and are sent to the OLT 51 without a collision.
Initially, the ONUs 53 do not recognize the LLID, which is necessary for sifting through frames at the time of a downlink, and the timing of uplink transmission. Therefore, it is necessary to perform processes for establishing a bidirectional line called Discovery and Ranging. In those processes, a protocol called MPCP (Multi-Point Control Protocol) is used.
FIG. 6 is a chart showing a conventional discovery process in a GE-PON.
The OLT transmits a Discovery Gate message (Discovery Gate) to ONUs periodically. A LLID for broadcast (Broadcast LLID) is assigned at this point.
Upon receiving of the Discovery Gate message, each ONU waits for a random time (d) so as to avoid a collision at the time of an uplink, and then returns a Register Request message (Register Request).
The OLT determines the LLID to be assigned to the ONU having returned the Register Request message, and notifies the ONU of the LLID through a Register message (Register). The OLT also measures the distance from (or performs ranging on) the time stamp in the received Register Request message to the ONU.
The OLT then determines the timing of the next uplink (the transmission start time and transmission band of the ONU) and notifies the ONU of the timing of the next uplink through a Gate message (Gate).
Based on the received Gate message, the ONU returns a response message (Register Ack). At this point, the discovery process comes to an end.
In the above manner, when an ONU is connected to the OLT, the OLT can automatically detect the ONU, and establish a communication link by assigning a LLID to the ONU.
A GE-PON has advantages such as low costs, no power sources, and simultaneous discovery processes by broadcast, because a passive device called a splitter of a passive device is used. On the other hand, the optical power is divided at the splitter, and the divided optical power decreases as the number of the division increases. Therefore, there is a trade-off relationship between the number of subscribers that can be accommodated by one OLT and the maximum distance to a subscriber. Accordingly, there is a theoretical limit to the maximum number of subscribers accommodated by one OLT or the extension of the maximum distance. Also, since all optical signals reach all ONUS in a GE-PON, there is a limit, in principle, to on-line confidentiality.
As an active optical access system using an optical switch instead of a splitter, a GE-OSAN (Gigabit Ethernet—Optical Switched Access Network) is disclosed in a document (Non-Patent Document 1).
In a GE-OSAN, IEEE 802.3ah is used basically, and the optical switching unit includes a 1:9 splitter and an O/E (optoelectronic) converter in an optical switching device provided between the OLT and the ONUS. The optical switching unit refers to the LLIDs in the preambles of all the Ethernet (registered trade name) frames, and switches on and off an optical switch. In this manner, a discovery process is realized without broadcast.
Patent Documents 1 and 2 also disclose techniques related to GE-OSAN.